Why 1+2 Doesn't Equal 3
Many people who embark on a fitness and health journey expect to see results that mirror their efforts every single day. The truth is, weight loss (or gain) is not a straight line—there will be many curves, highs and lows. Some days you might see the numbers on the scale going down. Some days they will go up, and some days they will not change at all even when you feel you've worked really hard on your fitness and nutrition.
For many of us the scale can be a dangerous tool when we are not familiar with all the factors that can affect weight changes: weight can fluctuate from one day to the next, and when we attach emotions to undesirable changes we think we see (for example, if we are trying to lose weight but the numbers on the scale are going up, or if we're trying to gain but the scale tells us we've lost some) we are adding stress to the process, which in turn can affect these numbers further...
Weight fluctuations don't always mean you've gained fat. Weight consists of how much food you have in your stomach, how much water you’ve drunk, how much your bones weigh, how much your muscles weigh, and how much water your body is retaining (which can be due to eating more carbs and sodium than usual and even physical/mental/emotional stress).
If you are training at the gym as well, keep in mind that over time you will gain some weight that is not fat: your bones will become more dense, your muscles will grow and add to your overall weight, which really is what you see on the scale. If you don't get enough sleep or have more stress in your life, it can also cause fluctuations on the scale, as can hormonal changes or a woman's monthly cycle.
Understanding this, try to look at your overall trend in a certain amount of time (an 8-week period for example), instead of getting attached to the numbers on a daily basis. Stay consistent, keep giving it you're best effort, keep crushing your workouts and tweaking your diet and you'll reach your goals!